There are already known various arrangements for removing material from workpiece surfaces, either to give such surfaces their desired configurations in the first instance (such as by turning, milling or grinding), or to improve (such as by fine grinding, honing, polishing or lapping) the surface quality that has been originally obtained in some other way. Such surface quality improvement usually encompasses not only the smoothness of the surface in question, but also its conformity to the desired configuration.
One field of human endeavor in which it is particularly desirable if not mandatory to obtain a very high surface quality is the optical field where any surface irregularities or deviations from the desired surface contour, no matter how minute, would invariably adversely affect the quality of the light reflected or refracted at such a surface. Therefore, it is customary in the optical field to resort to a lapping and/or polishing operation as the last step in the manufacture of optical components.
All currently used optical polishing processes are based on the principle of wear in that usually a polishing compound consisting of fine particles, for instance, metal oxide particles, which are mixed with a liquid carrier is introduced between the optical surface to be polished and some kind of a lapping material. Classically, such optical polishing has been carried out using pitch lap elements constructed of or carrying wood rosin or coal tar pitches, into which the particles contained in the polishing compound can be embedded as a function of time during the actual polishing process. After the introduction of the polishing compound, the pitch lap element which consists of or is provided with the lapping material is pressed with a constant pressing force against the optical surface to be polished or otherwise treated and is then caused to move in a series of oscillating translatory and/or rotating motions over the optical surface. As this occurs, the polishing particles that have become embedded in the lapping material, in turn, remove material from the optical surface due to a currently not well understood combination of mechanical abrasion, thermal flow and chemical attack.
This relatively unsophisticated approach employing a constant pressing force works reasonably well when the surfaces to be treated are planar or have another relatively simple geometry, such as spherical. This is so because the surface of the lap element that is juxtaposed with the region of the surface being treated can be relatively easily conformed to the desired configuration of the surface being treated and it then exerts different local pressures on different points of this region, the magnitude of each such local pressure being dependent on the extent of deviation of the affected point from its desired location, so that material is removed from more elevated points of the optical surface being treated much more rapidly than from less elevated points, until the differences between the peaks and valleys of the optical surface are either eliminated or reduced to an acceptable value. Thus, ideally, this approach should improve not only the RMS (root mean square) roughness of the affected surface, but also the conformity of such surface to its desired shape.
However, experience has shown that even under these relatively simple circumstances the final quality of the thus treated surface, and especially the conformity of such surface to its desired overall configuration, leaves much to be desired. One of the reasons for this less than ideal situation is that, in the equation which is widely believed to govern the polishing process and which postulates that the amount of material removed at any point of the optical surface is proportional to a proportionality constant times the local pressure at that point times the instantaneous velocity of the lapping surface over the surface being polished, the proportionality constant is actually a variable which is a complex combination of some forty-three parameters, some of which are the lap wear, viscosity changes, temperature changes, particle size distribution, particle chemistry, and others. Inasmuch as many of these parameters can change constantly and unpredictably not only as the polishing operation proceeds but also from one point of the surface being treated to another, it is very difficult if not impossible, to choose the pressing force in such a manner as to achieve a predictable wear of the material from the surface being polished under all conditions, even when the effected surface has a relatively simple configuration, such as one of those mentioned above. Obviously, this problem is further exacerbated when the surface to be polished has or is to obtain a more complex configuration.
A relatively recent development in the age-old art of optical polishing is the use of computer numerically controlled (CNC) machines. Such machines render it possible not only to smooth or polish the surface in question, but also to improve its conformity to the desired overall shape, be it planar, spherical, or curved in any other manner. This is accomplished by removing material from the surface being treated to different depths at different regions of such surface. This shaping or figuring of the optical surface, as it is often called, is normally accomplished by comparing the actual shape of the optical surface as it exists prior to the polishing operation with the desired final shape of the optical surface. By using the difference between these two values for any point on the optical surface as an indication of the amount of material to be removed, it is possible to achieve the desired overall shape of the optical surface. Such a polishing machine then uses this difference function as the primary input information for determining paths and dwell times for the polishing lap. Typically, an embedded computer model of the CNC machine convolves the wear function of the polishing lap with the aforementioned difference function to determine, for example, the required dwell times and velocities.
The above-discussed pitch laps have been applied to computer controlled surfacing, but only with moderate success because the wear rate of such pitch laps changes constantly with time and temperature. In addition, the quality of the optical surface, that is, the RMS roughness and the resultant scatter pattern, changes as a function of time. Unfortunately, when using the conventional pitch laps, the changes in the pitch lap performance are not easily predicted, nor are they easily made repeatable so that they could be programmed into the computer and compensated for by the associated control system of the CNC machine. The end result of this is that computer numerically controlled polishing, as practiced heretofore, could predict or control the final shape to no better than five percent. While this may be acceptable since these errors may be reduced by using a number of iterative runs, such a process is slow, time consuming and costly.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide an arrangement for improving the surface quality (shape and finish) especially of optical surfaces, which arrangement does not possess the disadvantages of the known arrangements of this kind.
Still another object of the present invention is to develop the arrangement of the type here under consideration in such a manner as to achieve accuracy, predictability and repeatability in the shaping and smoothening of the affected surface.
It is yet another object of the present invention to devise an arrangement of the above type which would render it possible to accomplish the material-removal operation in a very expeditious manner.
A concomitant object of the present invention is design the arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.